The present invention relates to a method for processing data pertaining to an activity of partial electrical discharges taking place in a three-phase electric machine powered with square wave voltage, said data comprising a plurality of discharge signals and a plurality of sets of three values of power supply voltage of the machine, each set of three voltages being measured in concurrence with a corresponding discharge signal. In particular, the present invention relates to a method for evaluating for diagnostic purposes an activity of partial electrical discharges in a three-phase electrical motor powered with square wave, e.g. by means of an electronic power converter operating in PWM logic.
Electronic power converters enable to control induction motors in an extremely effective manner, allowing to transform electrical energy into mechanical energy with great flexibility. On the other hand, the voltage wave forms which converters impose to the terminals of the motor have proven to be extremely harmful with respect to the duration of the insulation, particularly in motors built with enameled wire windings.
The research conducted throughout the world has shown that the electrical stresses at the terminals of motors driven by converters can be considerably high, causing the phenomenon of partial discharges (PD). PDs are discharges of small amplitude that do not short-circuit the electrodes, but cause a progressive degradation of the insulating system. This degradation process can be particularly rapid in deteriorating organic dielectrics like the enamels used in the insulation of the motor wires. Relatively slower is instead the degradation of dielectrics containing inorganic materials (mica, titanium dioxide) in an organic matrix (resin or polymeric paint). In any case, the possibility of verifying whether partial discharges occur or not in a motor driven by a converter, and of quantifying any such phenomena, has great importance for diagnostic purposes, or to prevent failures during operation.
However, the measure of the PDs in motors driven by means of converter and, above all, the subsequent assessment of the results of the measurements present some problems.
In the first place, particularly in new-generation, low voltage converters, the up ramps of the voltage wave forms may have a considerable slope, in the order, for example, of tens of kV per microsecond. These wave forms of the power supply voltage generate interference constituted by signals having a high frequency content and can conceal the presence of the partial discharges, if the sensors used for the measurement are not appropriately selected.
To the problem of measuring the signals associated to the PDs is added the difficulty in recognising any interference, and in interpreting and rationalising the results of the measurement itself.
In this light, it should be noted that, in order effectively to assess for diagnostic purposes the signals pertaining to a partial discharge activity, it is important to relate each measured discharge signal to the voltage that produced it. Relating a signal in a PD measurement to the voltage that produced it has two fundamental purposes.
A first purpose is to determine, within a three-phase machine, which insulation section is affected by the discharge.
A second purpose is to verify whether the measured signals are correlated with the power supply voltages (which generally have a substantially periodic profile). In this way, it is possible to discriminate the measurement noise from partial discharges and to verify the integrity of the insulating system.
To date, no systems are known for systematically relating a signal measured in a measurement of partial discharges to the voltage that produced it in the case of measurements performed on three-phase machines powered with square wave (e.g. with PWM logic), especially in the cases in which the signals of partial discharges are measured by means of a single sensor, shared by the three power supply phases (antenna).
Known techniques provide for assessing the measured signals as a time succession of events, with no possibility of relating them effectively to the power supply voltages. Therefore, all subsequent statistical processing of the measured data has little significance, if not altogether erroneous, because it is based on the processing of a heterogeneous set of data, comprising discharge signals pertaining to different portions of the insulating system (i.e. corresponding to discharges produced by voltages of different phases) together with signals relating to interference.